Adding Brightness values to the Print module is a band-aid, and yes, adding +10 is an indication that the system pipeline is way off.
I repeat, in my case, nothing changed other than the OS (Mojave) and Lr8.x. Same MBP Retina display and LG 5k. And if anything, my observation is the opposite of the typical reports - people complain their prints are dark; in my case they are slightly lighter. I suspect that some brightness level changed in OS X Mojave and/or Lr 8.x. But as i said, it is a tiny change, so I can correct by increasing very slightly my screen brightness.
Lr Print module +10 brightness correction as used by a LuLa Master... Ouch!
Thanks for the comments!